1. Field of the Invention
The present invention relates to a method of correcting image data which is employed when an image is printed, and more particularly to a technique for preventing deterioration in the image quality which occurs when a correction process is performed.
The present application is based on Japanese Patent Application No. Hei. 10-4081, which is incorporated herein by reference.
2. Description of the Related Art
In recent years, printing apparatuses, such as thermal transfer printers and laser beam printers, have widely been used as image output terminals for personal computers and work stations. Printing apparatuses of the foregoing type structured to output image data, such as characters and line drawings, must be capable of accurately and quickly outputting an image having multiple gradation.
Therefore, an image has usually been output such that original image data is subjected to a plurality of correction processes to produce final image data which must be printed.
When an image is printed by a printing apparatus of the foregoing type, a command to output an image is issued from an application which is being executed by, for example, a client computer. In accordance with the command to output an image, image data which must be printed is subjected to a correction process and a dither process. That is, the correction process and so forth are basically performed with the gradation accuracy (for example, 256 levels) of original data.
FIG. 26 shows a block diagram showing steps of an image correction process according to the related art. Referring to FIG. 26, a RIP machine performs density calibration correction 261. A printer engine performs correction of irregular densities in a main scanning direction, a process 262 for correcting the difference between the temperature of the right portion of the head and that of the left portion of the head and a multivalued dither process 263. Thus, when strobe width conversion 264 is performed, correction including correction of the difference in the environmental temperature has been performed.
When the correction process is performed with the gradation accuracy of original data, the correction accuracy is however limited by the gradation accuracy (the number of gradation is 8 bits, that is, 256 gradation). Therefore, the correction accuracy has a limitation to {fraction (1/255)}. Hence, the steps of the densities of printing caused from gradation skip of {fraction (1/255)} sometimes appear on the corrected image.
The foregoing gradation skip can be prevented when the number of gradation of image data is enlarged from, for example, original 256 levels to 1024 levels. Moreover, the 1024-level image data is subjected to a correction process. Thus, the gradation skip can be prevented. Although the gradation skip can be prevented if the number of the gradation is enlarged, the line-type head must be controlled delicately. In the foregoing case, there arise problems in that the image reproducibility deteriorates and in that the image recording speed when the image is formed is reduced.
In view of the foregoing, an object of the present invention is to prevent gradation skips caused from a data correction process which is a combination of a correction process, which is performed by a printer engine, and a multivalued dither process without a necessity of improving the control accuracy of the line-type head and reduction in the recording speed.
To achieve the above-mentioned object, according to one aspect of the present invention, there is provided an image correction method structured such that supplied image data is converted into image data which must be output when supplied image data is output by a printing apparatus including a line-type head, the image correction method comprising: a step for CMS-converting supplied image data in three dimensional or higher order; a step for increasing gradation accuracy of image data after the CMS conversion has been performed; a step for performing a process of correcting image data, the gradation accuracy of which has been increased; a step for performing a multivalued dither process; and a step for converting processed image data into a strobe signal with which the line-type head is controlled.
As a result, image data having an increased gradation accuracy can be subjected to the correction process. Since the gradation width can be fined, correction errors can be decreased. Therefore, the gradation skips caused from the data correction process can considerably be reduced.
It is preferable that the correction process includes any one of main-scanning-directional density irregularity correction for correcting irregular density in a main scanning direction of the line-type head, density calibration correction, and head right-and-left temperature difference correction for correcting change in the printing density occurring by dint of the temperature difference between the right and left of the line-type head.
It is preferable that the multivalued dither process is performed to record the gradation by converting input image data into recording dots having different sizes, and the conversion of the density level of image data into the size of the recording dot is performed such that at least five lines of characteristic curves of a gradation conversion table indicating the relationship between gradation values and energy for forming the recording dot are set, at least three types of energy values which are not maximum or minimum energy at a predetermined gradation value are set for a half tone image, and the number of printing points having substantially minimum energy is set to be not smaller than half of the number of all of printing points at a minimum gradation value at which a printing point having substantially maximum energy exists.
The image correction method according to the present invention has the structure that at least five lines of characteristic curves of a gradation conversion table indicating the relationship between gradation values and energy for forming the recording dot are set. Moreover, at least three types of energy values which are not maximum or minimum energy at a predetermined gradation value are set for a half tone image. Therefore, smooth continuous gradation value can be obtained. Moreover, the number of printing points having substantially minimum energy is set to be not smaller than half of the number of all of printing points at a minimum gradation value at which a printing point having substantially maximum energy exists. Therefore, even if a low density pixel is recorded on white portion, rough surfaces can considerably be reduced.
The description xe2x80x9csubstantially maximumxe2x80x9d means a structure that the energy of the characteristic curve at the maximum gradation value at each color which must be printed is raised as compared with the energy of the characteristic curve at the other gradation value. The foregoing structure is effective to improve the collapse characteristic of a xe2x80x9csolidxe2x80x9d image.
It is preferable that power supply voltage which is supplied to the line-type head is corrected in accordance with the atmosphere including the temperature and humidity.
It is preferable that a plurality of gray charts including a first gray pattern formed by C, M and Y and a second gray pattern formed by K in such a manner that a required color tone of the first gray pattern is realized and corresponding to different densities are prepared, the density of each gray chart is read by a color scanner to measure characteristics of reflection intensities with respect to wavelengths, and a CMS table to which a reference is made when the CMS conversion is performed is corrected in accordance with the obtained characteristics of the reflection intensities.
It is preferable that the main-scanning-directional density irregularity correction is performed such that an elongated pattern having a predetermined gradation value is printed in the main scanning direction of the line-type head, aligning a main scanning direction of a line sensor to a sub-scanning direction of the line-type head, and the line sensor is relatively moved in the main scanning direction of the line-type head so that the density of the printed elongated pattern is detected, correction conditions for the positions of pixels are obtained in accordance with the detected densities of printing and the predetermined gradation value, and image data which must be output is corrected in accordance with the correction conditions.
When the above-mentioned image correction method is combined with a recording material having a thin ink layer, a further satisfactory effect can be obtained. The foregoing recording material is exemplified by the following material disclosed in Unexamined Japanese Patent Publication No. 7-117359.
The recording material is a thermal transfer recording material is employed which includes a substantially transparent thermosensitive ink layer which contains 30 parts by weight of pigment and 25 parts by weight to 60 parts by weight of organic amorphous high molecular-weight polymers having a softening point of 40xc2x0 C. to 150xc2x0 C. and which has a thickness of 0.2 xcexcm to 1.0 xcexcm, 70% or more pigment in the thermosensitive ink layer has a particle size of 1.0 xcexcm or greater, and an optical reflection density of a transferred image on a white support member is not smaller than 1.0.